Compton imaging is a known technique for gamma ray detection that can provide both energy and spatial resolution for gamma rays in the energy range of hundreds of keV to tens of MeV. By detecting kinematically-linked pairs of Compton scatterings and photo-absorption events, both the energy and the trajectory of individual detector-incident gamma rays may be deduced. A Compton imaging detector can be simultaneously sensitive to gamma rays of many different energies, arriving from many different directions.
The basic operating principle of a Compton imager is illustrated in FIG. 1. A gamma ray 100 encounters a scintillation material 106 of the detector where it undergoes Compton scattering in a first interaction 102 and is then absorbed in a second interaction 104. The unknown energy of the incident gamma ray 100 is denoted by Eγ, and the unknown incident scattering angle is denoted by φ. A Compton imager attempts to determine Eγ and φ from measurements of the energies deposited in the two interactions, denoted by E1, and E2. Specifically, the incident energy Eγ of the gamma ray and the Compton scattering angle φ can be determined from the expressions
            E      y        =                  E        1            +              E        2              and                    E        2            =                                                  E              y                        ·                          m              e                                ⁢                      c            2                                                              E              y                        ·                          (                              1                -                                  cos                  ⁢                                                                          ⁢                  φ                                            )                                +                                    m              e                        ⁢                          c              2                                            ,  where mec2 is the rest energy of the electron (i.e., 0.511 MeV).
In addition to measuring E1 and E2, a Compton imager also needs to measure the corresponding locations (x1, y1, z1) and (x2, y2, Z2) of the two interactions. From these locations, the imager can then determine the trajectory 108 relative to which the scattering angle φ is defined, and thereby determine the orientation and size of a cone 110 whose surface is the collection of all points from which the gamma ray may have originated.
Using knowledge of the energy Eγ and the Compton scattering angle φ for multiple incident gamma rays, a source of gamma radiation can be located, as shown in FIG. 2.
The events can first be partitioned into separate bins according to their energy. For each energy bin, the intersections of the cones are calculated. FIG. 2 shows a two-dimensional cross-section of the cones for events in a single bin, i.e., events having nearly the same energy. For simplicity of illustration, the conic sections are all shown as circles. The solid circles, such as circle 202, intersect at a single point 200, which indicates the location of the source, e.g., inside a vehicle 208. A few other circles, such as circle 204, are randomly distributed and represent stray signals, e.g., due to background radiation. The square 206 represents the detector viewing area.
The performance of a Compton imager depends on many factors including its gamma ray scattering and absorption efficiency and the precision with which it can measure the energies and locations of the gamma ray interactions. These, in turn, all depend on the specific design of the Compton imager, such as the scintillation material used, the technique used for localizing the interactions, and the method used for measuring the energies of the interactions.
Liquid xenon is an advantageous material for many Compton imaging applications because it combines an energy resolution between CdZnTe (CZT) and sodium iodide, the cost of sodium iodide, the position resolution of CZT, and the scalability of organic scintillator. Accordingly, researchers have developed various designs for Compton imaging devices using liquid xenon as the scintillation material. However, there remains a need for Compton imagers that provide better energy and angle resolution than these existing designs.